The present invention relates to simultaneous oxidation of SO2 to sulphuric acid, NOx to nitric acid, and metallic mercury vapour to a mercury chloride, in a flue gas, thereby allowing capture of mercury in the downstream process solutions.
U.S. Pat. No. 4,619,608, granted Oct. 28, 1986, describes the effects of sulphur dioxide and nitrogen oxides released from the burning of fossil fuel.
Regulations in force since 1985 have mandated two stages of cleanup for the flue gases emitted from fossil fuel powered utility power stations. A December, 1996 proposal by the U.S. Environmental Protection Agency (EPA) has proposed even higher control levels based on health concerns.
In addition, mercury has now become a pollutant of concern. Costs of capturing mercury can run as high as $100,000 per pound using activated carbon or limestone systems. A problem with the limestone systems is the re-release of the mercury once captured.
In February, 1998, the EPA released a mercury report to U.S. Congress which identified the health concerns and the need to develop new technologies to capture mercury from the vapour emitted from coal fired power plants.
U.S. Pat. No. 4,619,608 describes a process for injecting chlorine to oxidize SO2 and NOx components of a flue gas stream, and removing them from the flue gas.
This invention is an improvement on that process and adds the capacity of trapping metallic mercury in a form that avoids the potential for re-release of the mercury. Costs to operate any system are a concern, and the process herein described is more economical than existing technology. The innovation is in the combination of the capture of all three pollutants of concern, namely SO2, NOx and Hg vapour, in one single economical process.
The invention is directed to a process for oxidizing gaseous pollutants in a flue gas stream composed of flue gases, water vapour and one or more gaseous pollutants selected from the group consisting of SO2, NO, NO2, H2S, and mercury vapour, said oxidized form of the pollutants being more readily removable from the flue gas stream by water absorption than the non-oxidized form thereof, comprising: (a) injecting sufficient chlorine in a gaseous form, a liquid form, or as a chlorine water solution, into the flue gas stream while the flue gas stream is at a temperature greater than 100xc2x0 C. to thereby react the chlorine with the pollutants and permitting the flue gas stream/chlorine mixture to react for a sufficient time to enable a significant amount of oxidation of the pollutants to occur, whereby an oxidized flue gas stream comprising flue gases, water vapour and one or more gaseous oxidized said pollutants is formed; (b) scrubbing the oxidized flue gas stream with water, or water solution, of pH less than or equal to 7; and (c) adding sufficient alkali metal iodide salt to precipitate mercuric iodide from the water or water solution of pH less than or equal to 7, as the concentration level requires. The alkali metal iodide salt can be potassium iodide.